Mine's 13mm, which isn't great considering some are 16mm or even thicker than that - but my bike is either between my legs (not likely to be stolen), round the back at work (again, not likely to be stolen but I still lock it up as another business backs onto it and could easily nick it if it was just left there) or in the shed at home.
I can't get my D-Lock around a lamp post which irritates me, I can only do that with my cable lock (and I don't wanna use a £9 cable lock from JJB on a bike that cost £300) so I'm finding myself stuck for places to lock it when I'm at work if I can't put it round the back.
I suggest buying 2 mini D-locks as well if you have quick-release wheels - some thieves will just nick the wheels (especially the front one, easy to get off) just to piss people off.
